The avalanche photodiode (APD) is a semiconductor device that can detect extremely low levels of electromagnetic radiation. An APD is constructed so that an electron dislodged by a photon will hit other atoms in the silicon of the APD with sufficient velocity and energy so that additional hole-electron pairs are created by the collisions. Typically a free electron will create a number of hole-electron pairs, and the electrons from these pairs will, in turn, create additional electrons, thus creating an "avalanche" process. This multiplication of electrons gives the APD an effective gain and allows detection of very low light levels.
One potential use for APD's is in detecting the output of scintillating crystals used to detect gamma rays in such fields as high energy physics experiments, nuclear medicine, and PET cameras. Currently, photomultiplier tubes (PMT's) are the principle device used to detect scintillator outputs. APD's would have many advantages over PMT's due to their higher quantum efficiency, small size, and insensitivity to external magnetic fields. In applications such as PET imaging, the ability to create large arrays of APD's with much finer positional resolution than PMT's would be advantageous.
The output wavelengths from the most commonly used scintillators is in the range of 350 to 550 nm, and currently known types of APD's capable of detecting light at these wavelengths have disadvantages which render them of limited suitability for use as scintillator detectors. FIG. 1A illustrates a conventional reach-through APD. The substrate is a p.sup.- -type layer 4. The p-n junction is created by a diffusing more highly-doped p-type layer 6 into the bottom surface of the APD, followed by a shallower n-type layer 8. Due to the higher doping of the p-type layer 6, the high field is concentrated in a narrow region surrounding the junction of layers 4 and 6. The value of the electric field over the path denoted by the line a-b in FIG. 1A is shown in FIG. 1B. These types of APD's are made in a variety of sizes and thicknesses.
The reach-through APD is characterized by a relatively low-field drift region 10, shown by the low field strength plateau on the left of FIG. 1B, followed by a narrow, high-field multiplying region within the p-type layer 6, represented by the spike at the right in FIG. 1B. Photons impinging on the top surface 2 create hole-electron pairs. Even though the field in region 10 is much lower than in the narrow multiplying region, electrons and holes which are created in this region are still collected rapidly, and charge collection in this device is fast. However, no multiplication occurs in this region. When an electron enters the high field in the multiplying region, its energy increases to the point where it has a significant probability (called the ionization coefficient) of knocking loose another electron. The two electrons, and to a lesser extent the hole, can then have further ionizing collisions. This process is generally referred to as avalanche gain.
The prior art reach-through APD structure shown in FIG. 1A suffers from the disadvantage that any thermally generated electrons created in area 10 are multiplied by the same gain factor as electrons created by impinging photons, resulting in high background noise. Due to the width of this region, typically in the range of 30 to 150 .mu.m, significant noise is produced by thermally-generated hole-electron pairs. A second disadvantage of this structure is that achieving a low value of k.sub.eff (a weighted ratio of the ionization coefficients of holes to electrons) requires a very long and expensive diffusion. A third disadvantage is that reach-through APD's are very thin, typically 150 .mu.m or less in thickness, and are generally fabricated using small silicon wafers of two inches or less to reduce breakage to an acceptable level. Most automated wafer handling systems are set up to handle conventional four-inch silicon wafers and cannot be used to fabricate these types of APD's. This keeps manufacturing costs high. The delicate nature of these APD's also make them less suitable for fabricating large array structures.
Another type of prior art APD is the deep-diffused, beveled-edge APD using neutron-transmutation-doped silicon, shown in FIG. 1C. In these diodes, a p-type top layer 10 is deeply diffused into an n-type substrate 14. An n.sup.+ diffusion provides an ohmic contact on the bottom of the APD. The depletion region extends between the dotted lines 18. Hatched region 15 is the multiplying region in which the depletion field is high enough to cause gain. Due to the relatively low value of the maximum field in the multiplying region, the k.sub.eff values for this type of APD can be made relatively low.
These APD's suffer from several disadvantages, however. They require very high bias voltages, on the order of 1500 to 2500 volts, which makes circuit design more difficult and creates problems with breakdown along the edges of the device. The beveled edge is designed to reduce the field strength along the edge of the diode to prevent breakdown. Even so, it is difficult to passivate the beveled edge using conventional techniques. This increases manufacturing expense and decreases the reliability. The requirement for a beveled edge makes fabrication of arrays difficult due to the need to cut into the bottom of the device to make the beveled edge. Additionally, since the depletion layer 18 does not extend to the surface of the APD, collection of primary carriers is slow which increases timing uncertainty and the number of false background counts in applications where the APD is gated during intervals when photons are being collected, such as in PET imaging.